An IP phone, and the like, has recently been equipped with VAD (Voice Activity Detection) as a function for detecting presence/absence of a sound. Many phones are equipped with DTX (discontinuous transmission) as a function for not transmitting sound information during silence (see; for instance, Non-Patent Document 1 and Non-Patent Document 2). It is possible to decrease the amount of information to be transmitted (an average bit rate) by adoption of a configuration (hereinafter called “silence suppression”) that does not transmit sound information during silence. However, performance of silence suppression raises a problem of a break arising in the start of a sound when there occurs a shift from silence to presence of a sound.
Accordingly, there has been proposed a sound compression method for temporarily storing a picked-up sound in memory and reading the past sound from the memory when a change from silence to a sound occurs and transmitting the thus-read sound, thereby preventing occurrence of a break in sound, which would otherwise arise at start-up (see; for instance, Patent Document 1).
Non-Patent Document 1: ITU-T G.711 Appendix II to Recommendation G.711 (02/2000)
Non-Patent Document 2: RFC3389 Real-time Transport Protocol (RTP) Payload for Comfort Noise (CN)
Patent Document 1: JP-A-2005-266411